Electron beams utilized in various analytical instruments may be generated by one of two fundamental electron emission processes: thermionic emission and field emission. The two emission processes are not exclusive, and some instruments use both phenomena, as, for example, electron microscopes which use the thermal field mode of operation.
Thermionic emission has been the most common mechanism by which electrons are liberated from a cathode. In thermionic emission, the cathode is heated to transfer energy to upper energy-level electrons, providing sufficient kinetic energy to them to allow them to escape the surface barrier of the solid cathode. Once the electrons leave the surface of the cathode, they may be accelerated and focused by electron lenses.
The thermionic emission process has been most commonly used in the past because of the simplicity of thermionic cathode construction, relatively long cathode lifetimes, and high total current capabilities. The primary disadvantage in analytical instruments of the thermionic emission process is the inherently low current density (low brightness) of the beam emitted from the cathode.
Electron beams obtained from field emission sources generally have an inherently high current density (high brightness) but a relatively low total current capability. The construction of field emission cathodes is more difficult than for thermionic emission cathodes, and the lifetimes of field emission cathodes are generally shorter than the lifetimes of thermionic cathodes.
Electron field emission guns have been developed for use in practical analytical instruments requiring finely-focused, primary electron beams, such as electron microscopes. Beam diameters as small as a few tens of angstroms have been obtained by operating such instruments at very high energies, in the range of one to two hundred thousand electron volts. Field emission processes are naturally suited to use in such high energy instruments since high voltages are normally required to obtain field emission.
High energy electrons have a relatively large penetration depth into materials and cause greater damage to the material than low energy electrons. If surface information concerning a sample is desired, or if sample damage is to be minimized, it is more appropriate to utilize a low energy electron probe. For example, the penetration depth of a 100 eV electron is only a few atomic layers in most solids. Despite the need for low-energy electron probe systems, the development of practical low-energy instruments has proceeded more slowly than high-energy instruments because of the difficulty encountered in constructing low energy electron sources and lenses which nonetheless have high resolution. Although the ultimate spatial resolution attainable using low energy electron beams is less than that attainable with high energy beams, due to a greater space-charge interaction time associated with low energy beams, substantial improvements in resolution over that attainable with present instruments is possible.
The field emission phenomenon requires very high electric field intensities at the emission surface, in the range of 10.sup.8 volts per centimeter. It is possible to increase the electric field intensity at an emission surface by increasing the magnitude of the voltage applied between the emission surface and an adjacent electrode or by decreasing the separation between the emission surface and the reference electrode (anode). Electric field intensity can also be increased by decreasing the radius of curvature of the emitting surface. Thus, a sharp emission (cathode) tip and a high extraction voltage between the cathode and a reference anode are commonly employed in practical field emission instruments. The emission tips are usually electrochemically etched from small diameter wire so that the radius of the end point of the tip is a few hundred angstroms.
A typical configuration for a field emission electron gun includes a flat, apertured extraction anode mounted adjacent to a sharp emission cathode tip located opposing the aperture of the anode. The kinetic energy of the electrons emitted from the cathode will be essentially that of the extraction voltage applied between the cathode and anode, usually 5 keV to 10 keV. The energies imparted to electrons by these extraction voltages present no problem in high energy applications, as, for example, transmission electron microscopy, where the electrons must typically be further accelerated to the operating energies on the order of a hundred thousand electron-volts. If, however, an electron beam is to be used in low-energy applications, e.g., surface analysis or analysis of materials sensitive to electron beams, a high extraction voltage is a distinct disadvantage. In low-energy applications, the field emitted electrons must be decelerated with an appropriate lens system before reaching the target sample. The deceleration process tends to spread the electron beam spatially for three reasons. The first is that all electron lenses have aberrations such that a perfectly parallel beam entering the lens will be distorted, with some electrons sent off into directions other than the desired direction. The second reason is that the field-emitted beam is neither perfectly chromatic (mono-energetic) nor perfectly parallel, and thus the lens power will not be correct for all of the electrons entering the lens. The third reason is that electrons tend to repel each other, and the beam spreading due to this repulsion is more pronounced at low kinetic energies than at high energies. In general, the distortion that a lens introduces into a beam entering it will depend on how strongly the lens must interact with the beam. For example, if a lens is required to decelerate a beam from 5 keV to 100 eV and focus it, more beam spreading occurs than if the same lens is required only to decelerate a beam from 400 eV to 100 eV and focus it.
Thus, when a low energy beam is to be produced, it would be most desirable if the energy acquired by the beam in the process of field emission from the cathode were as low as possible, and therefore the voltage applied between the cathode and extraction anode should be as low as possible. The electric field intensity at the cathode may be increased, with the applied voltage kept constant, by decreasing the separation distance between the emission cathode tip and the anode, but it may be noted that the separation distance between such a tip and an apertured, flat-plate anode cannot be reduced indefinitely to increase the electric field intensity at the cathode. This is so because of the finite size effect of the aperture; as the emission tip comes close to the walls in the anode defining the aperture, the anode no longer approximates a large, flat conducting surface and will not generate sufficient electric field strength at the emission tip surface to allow field emission. However, the aperture itself cannot be made arbitrarily small to correct this, since a decrease in the size of the aperture decreases the intensity of the beam because more electrons will be attracted to the plate and fewer will pass through the aperture.